I decided to see if it were possible to create a Yume Nikki MMO. I noticed various issues while creating it which may help others when making their own.

* Scalability: the rooms in Yume Nikki are small so fitting a lot of people in one room would require servers or duplicate rooms. This mainly happens in Madotsuki's room where only about 40~60 people can fit (unless you allow players walk on top of each other). You can solve this by making custom rooms or by isolating players.

* Isolation: Yume Nikki as a game evokes the feeling of isolation throughout the gameplay. This mood will obviously be destroyed if it becomes a MMO. This again could be solved by making specific rooms only for one player. By this I mean the room can be accessed by multiple users but the user will only see themselves in the room and not the other users.

* Identification: I initially intended on creating a chat system for other users to talk but then I realized that a method for identifying other users would require some kind of Identification. Like a name tag or something below their character. To create something like this you would either have to make a tiny font which would be barely legible. Or break the pixel rule and put an actual font in. This often breaks the aesthetics, here's an example: https://i.imgur.com/sjeJH1S.png

While other parts like making all the rooms and events work properly are difficult they are difficult in the sense that they are a tedious task rather than a technical difficulty. These were all the issues I could see.

Aside from being potentially detrimental to the feel of the game, another important thing to consider is user interactivity. The backbone of any mom is how users can actually interact, otherwise, there's no point. This seems like a novel idea that a few people will try for awhile beforehand getting bored and abandoning it; that result seems hardly worth the effort. The chat could just anonymous. If you do go through with this, here's two suggestions. Make the color of the other users look different than the made the player Is controlling. A race type of feature could also be used where at a given starting point, every user tries to beat the game as quickly as possible by triggering events and collecting effect. Being able to see the progress of other players during this would increase the tension. Other than that, I can't think of anything else you would be able to do with the idea.